Gear pumps won’t have a case drain. The side plate leakage just goes back to the inlet side of the gears, and the leakage past the end bearings has a drilled port taking it to the suction port of the main casting. If that port on the shaft end is blocked (very unusual) or not there (more unusual, but has happened) the pressure build up can blow the shaft seal.
Variable pumps, piston or vane, usually need a case drain as the leakage goes into the case. Some, but pretty rare, port the case to the inlet like the gear pumps do, but that is rare and risky. Too much case back pressure against the pistons can cause cavitation or lifting of the pistons, Expensive quickly.
I would rarely use a case drain filter on piston pump because of risk of too much back pressure. just a couple psi can damage pistons. If needed, it has to be grossly oversized, or fairly coarse (maybe 50 or 100 micron). Same story with suction strainers on piston pumps. Has to be a very thought through decison as they are risky.
Return filters are IMO essential even on a splitter. Yes, there are the ‘ran 40 years on used motor oil and no filters’ stories. I won’t even touch that topic.